I have ridden from San Diego to Phoenix on the ACA Southern Tier route. It took me 9 days and I both camped out and slept in motels. Much of the country between these two cities is desert. Some of it desolate. There is one stretch, between Brawley, CA and Blythe--about 90 miles, where there are no hotels and very few services, at all. I did most of it in one day but carried a gallon of extra water and camped on the ground just outside of hotel-less Palo Verde for the night.
I agree with staehpj1 that you shouldn't carry your gear on your back. A gallon of water weighs 8 pounds. Use a backpack if you want, but strap it to a rear rack.
I wrote a text-only journal of this ride, which started in Paso Robles, CA. You can read it here:
http://www.biketouringtips.com/bike....ike.report.htm
Good luck,
Ray